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Permeance and latex paint

user-6323476 | Posted in GBA Pro Help on

Martin et al.,

I have a question for a “Pro” on this blog. I am looking into different encapsulants or paints to go over lead paint in an older home remodel. The permeance for these latex based encapsulants varies widely.

In general, not considering the lead control aspects, would a lower permeance be better than a higher permeance? Since this is an older home, we are not talking about new construction materials, choosing a WRB, etc. Rather, we are trying to limit lead hazards and put a coating on the wall that will not create an additional problem of moisture build up in the wall which could lead to rotting or potentially encouraging molds.

I am considering a latex paint with a permeance of 2.894 perms by dry cup. This is just under the low range of most latex paints from a table I found at Building Science Corp. My thought is this would be a good choice as it would limit moisture moving through the paint, but not so much as to cause a moisture build up. The answer to this may also depend on climate/geography.

What do you all think about this?

Thanks,

Michael

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Michael,
    For the time being, I'll leave the lead hazard issue aside -- but you should know that painting over old lead-based paint may not reduce the hazard. There's a lot to say on that issue.

    In most cases, the vapor permeance of interior paint has little effect on wall system performance. That said, an interior vapor retarder (for example, a layer of vapor retarder paint) is required by most building codes in colder climate zones (Zones 5-8 and Marine Zone 4). The effect to which the presence or absence of this layer affects the moisture performance of the wall system is usually exaggerated.

    If you want more specific advice, you should (a) tell us where you live, and (b) describe all the layers of your wall assembly.

    For more information on these issues, see these two articles:

    Vapor Retarders and Vapor Barriers

    Do I Need a Vapor Retarder?

  2. user-6323476 | | #2

    Martin,

    Thanks for the reply. I live in Colorado. It's dry as you know, even in winter. The remodel being considered is an older home built in the 50's. I am not a builder so I cannot describe accurately the wall assembly. I am guessing it's fairly standard for Colorado. I am sure it does not have any of the new WRB type materials. The walls already have a layer of latex paint over the old lead paint with the exception of window and door frames.

    To my question, would an interior latex paint with low permeance create any types of water or mold problems vs. a latex paint with a higher permeance rating? Assuming adding a new coat of latex paint could improve existing conditions (reduce potential of water or mold issues), would the choice in paint permeance be different in a warm humid or cold humid climate?

    I'll start a new thread for the lead-based paint treatment question.

    Thanks,

    Michael

  3. GBA Editor
    Martin Holladay | | #3

    Michael,
    Q. "Would an interior latex paint with low permeance create any types of water or mold problems vs. a latex paint with a higher permeance rating?"

    A. No.

    Q. "Assuming adding a new coat of latex paint could improve existing conditions (reduce potential of water or mold issues)..."

    A. It cannot.

    Q. "Would the choice in paint permeance be different in a warm humid or cold humid climate?"

    A. No. If you are adding a layer of interior latex paint to a wall that already has at least two layers of paint, the permeance of the paint is fairly irrelevant. Each additional layer of paint reduces the permeance of the paint layer somewhat, but really -- vapor permeance isn't really an issue here.

    If you want to improve the moisture performance of your wall assembly, focus on air sealing -- not on the vapor permeance of the interior paint.

  4. Expert Member
    Dana Dorsett | | #4

    The vapor permeance of leaded paint is usually quite low, well under 1-perm. The permeance of any latex you applied over it will barely budge the needle- you might be able measure it under very controlled conditions, but in the real world the relative difference is nil.

    BTW: Having a vapor permeance specified to four significant digits "...2.894 perms..." ) is downright silly, since there is no way the sample to sample variance could be anywhere near that tight, even if painted in a laboratory with robots. In the real world the human-applied samples wouldn't be good to 2 significant digits, but it would probably be within 1.5 digits.

  5. user-6323476 | | #5

    Thank you Martin and Dana for your input on this potential project I am considering.

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